LT3081
12
3081fc
applicaTions inForMaTion
Table 1 lists many common output voltages and the clos-
est standard 1% resistor values used to generate that
output voltage.
Regulation of the output voltage requires a minimum load
current of 5mA. For true zero voltage output operation,
return this 5mA load current to a negative output voltage.
Table 1. 1% Resistors for Common Output Voltages
V
OUT
(V)
R
SET
(kΩ)
1
20
1.2
24.3
1.5
30.1
1.8
35.7
2.5
49.9
3.3
66.5
5
100
With the 50µA current source used to generate the reference
voltage, leakage paths to or from the SET pin can create
errors in the reference and output voltages. High quality
insulation should be used (e.g., Teflon, Kel-F); cleaning of
all insulating surfaces to remove fluxes and other residues
is required. Surface coating may be necessary to provide
a moisture barrier in high humidity environments.
Minimize board leakage by encircling the SET pin and
circuitry with a guard ring operated at a potential close
to itself. Tie the guard ring to the OUT pin. Guarding both
sides of the circuit board is required. Bulk leakage reduction
depends on the guard ring width. 50nA of leakage into or
out of the SET pin and its associated circuitry creates a
0.1% reference voltage error. Leakages of this magnitude,
coupled with other sources of leakage, can cause signifi-
cant offset voltage and reference drift, especially over the
possible operating temperature range. Figure 2 depicts an
example guard ring layout.
If guard ring techniques are used, this bootstraps any
stray capacitance at the SET pin. Since the SET pin is
a high impedance node, unwanted signals may couple
into the SET pin and cause erratic behavior. This will
be most noticeable when operating with minimum
output capacitors at full load current. The easiest way
to remedy this is to bypass the SET pin with a small
amount of capacitance from SET to ground, 10pF to
20pF is sufficient.
Configuring the LT3081 as a Current Source
Setting the LT3081 to operate as a 2-terminal current
source is a simple matter. The 50µA reference current from
the SET pin is used with one resistor to generate a small
voltage, usually in the range of 100mV to 1V (200mV is a
level that rejects offset voltage, line regulation, and other
errors without being excessively large). This voltage is
then applied across a second resistor that connect from
OUT to the first resistor. Figure 3 shows connections and
formulas to calculate a basic current source configuration.
3081 F02
SET PIN
GND
OUT
Figure 2. Guard Ring Layout Example of DF Package
Figure 3. Using the LT3081 as a Current Source
I
OUT
≥
5mA
V
SET
=
50µA
•
R
SET
I
OUT
=
V
SET
R
OUT
=
50µA
•
R
SET
R
OUT
IN
SET
OUT
+
–
LT3081
50µA
I
OUT
V
SET
R
SET
3081 F03
+
–
R
OUT
